A warp core casing composed of tricesium, with "Fantome" inside
Tricesium was a material that could be converted into an energy source.
In 2377, one of the ships that attacked the USS Voyager upon its entrance into the Void had a warp core casing composed of tricesium. When Voyager later found the ship crippled and its crew killed, they salvaged the casing and found "Fantome" hiding inside. (VOY: "The Void")